1. Compare the properties of electrons, protons and neutrons
2. What are the limitations of J.J. Thomson's model of the atom
3. What are the limitations of Rutherford's model of the atom?
4. Describe Bohr's model of the atom.
5. Compare all the proposed models of an atom given in this
chapter
6. Summarise the rules for writing of distribution of electrons in
various shells for the first eighteen elements.
7. Define valency by taking examples of silicon and oxygen
Sand
Answers
Answer:
1. Atoms are made of extremely tiny particles called protons, neutrons, and electrons. Protons and neutrons are in the centre of the atom, making up the nucleus. Electrons surround the nucleus. These three particles are compared with their properties and listed below.
2. Thomson's atomic model failed to explain how the positive charge holds on the electrons inside the atom. It also failed to explain an atom's stability. The theory did not mention anything about the nucleus of an atom.
3. he major limitation of Rutherford’s model of the atom is that it does not explain the stability of the atom. As we know now, when charged bodies move in circular motion, they emit radiations. This means that the electrons revolving round the nucleus (as suggested by Rutherford) would lose energy and come closer and closer to nucleus, and a stage will come when they would finally merge into the nucleus. This makes the atom unstable, which is clearly not the case. The electrons do not fall into the nucleus, atoms are very stable and do not collapse on their own.
It didn’t explain about the arrangement of electron.
4. Bohr’s model of the atom.
According to Bohr’s theory:
The atom consists of a small positively charged nucleus at its centre and surrounded by negatively charged electrons in a definite circular path.
The nucleus contains all the protons and neutrons of the atom.
The electrons of the atom revolve around the nucleus in definite circular paths known as orbits which are designated as K, L, M, N or numbered as n=1,2,3,4 outward from the nucleus. The orbits are also known as stationary orbits.
Each orbit is associated with a fixed amount of energy. The lowest energy level of the electron is called the ground state. Therefore, these orbits are also known as energy levels or energy shells.
5. THOMSON'S Model of An Atom :
(i) An atom consists of a positively charged sphere and the electrons are embedded in it.
(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
RUTHERFORD ' s model of an atom :
(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well-defined orbits.
(iii) The size of the nucleus is very small as compared to the size of the atom. (i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
6. the following rules are followed for writing the number of electrons in different energy levels or shells.
(i) the maximum number of electrons presents in a shell is given by the formula 2n
2
where
′
n
′
is the orbit number of energy level index 1,2,3... hence the maximum number of electrons in different shells are as follows :
First orbit or K-shell will be =2×1
2
×2 Second orbit or L shell will be =2×2
2
=8 third orbit or M shell will be =2×3
2
=18 fourth orbits or N-Shell will be =2×4
2
=32 and so on.
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8
(iii) Electrons are not accommodated in a given shell unless the inner shells are filled. That is the shells are filled in a stepwise manner.
7. Definition: Valency is defined as the combining capacity of the element. It is determined by the number of valence shell electrons present in the atom.
If the number of valence electrons is less than or equal to 4 so the valency is the same as the number of electrons present.
If the number of valence electrons is more than 4 then the valency is 8-(the number of valence shell electrons).
Example:
(1)Silicon has atomic number 14. The electronic configuration of silicon is 2, 8, 4. The number of valence electrons is 4. So the valency is 4.
(2)Oxygen has an atomic number 8. The electronic configuration is 2, 6. The valency is (8-6)=2.